1. Technical Field
Fatty acid esters of monovalent alkyl alcohols are used in manifold applications both as raw material and as intermediate products in the chemical and pharmaceutical industries. Moreover, such compounds are also in the food-processing industry, too, and have also been employed particularly as Diesel fuels in the recent past.
2. Prior Art
For the production of these compounds various ways have been proposed, starting out from crude oil or reproducing raw materials. In practical application, a particular importance must be awarded to the preparation of such esters from vegetable or animal oils and fats, not least for reasons of protection of the environment. As a rule, the methods so far applied are based on the base-catalyzed transesterification of fatty acid esters of polyvalent alcohols, particularly of fatty acid glycerides (cf., for instance, J. Am. Oil Chem. Soc. 61 (1984), p. 343, or Ullmann, Encyclopedia of Industrial Chemistry, 4th edition, vol. 11, page 432).
The industrial methods for the production of fatty acid esters of monovalent alkyl alcohols by base-catalyzed transesterification of fatty acid esters of polyvalent alcohols, which have been proposed in recent years, for instance those disclosed in the German Patent DE 3932514 or DE 4209779, employ partly a high expenditure in terms of apparatus and/or operate under cost-intensive conditions, i.e. at high temperatures and/or high pressures, and frequently also encompass complex processing steps such as distillations. For these reasons, such methods can be economically realized only at an industrial scale with quantities above 50,000 tons per year and more. These methods are not suitable for small installations producing 500 to 5,000 tons per year approximately. Methods specifically adapted to the potentials of such small installations are known, for example from the patents AT-386222, AT-397966, AT-387399, DE-3727981, DE-3020612, DE-3107318 or WO 92/00268. All of these methods are based on the aforementioned transesterification reaction and aim at a simplification going as far as possible, in an approach to reduce the costs and to permit an economic operation at a small scale as well. In particular, these methods try to desist from energy-intensive steps of separation, such as distillations, which are expensive and complex in terms of apparatus. Here the separation has been generally accepted as the established means of the first choice for the separation of the product or of intermediate products.
When non-refined starting oils are used, such as native oils, two important problems arise, however, with these methods. Native oils, specifically those of vegetable origin, contain, as a rule, slime substances such as phosphatides. These substances are surface-active and are therefore used partly as emulsifiers in the food-processing industry. In relation to the methods outlined here for the production of fatty acid esters, this characteristic furnishes the problem that these compounds take an expedient influence on the phase separation. Accordingly, a smooth realization of the method requires the use of starting oils from which the slime has already been removed, or the adoption of additional operating steps which require, as a rule, also supplementary parts of the installation for a reduction of yield losses induced by the incomplete or slow separation of phases. Fairly high levels of slime substances and particularly phosphatides in the starting oil give therefore rise to higher costs in conventional methods.
Moreover, the aforementioned oils often require also rather substantial quantities of free fatty acids whose presence takes a negative influence as well. As a matter of fact, these substances as free acids react with the basic catalyst added for transesterification, with the formation of soap. As a consequence, one part of the catalyst is neutralized and is therefore no longer available for the transesterification reaction. As a solution to this problem, it is possible to neutralize and/or remove the free fatty acids prior to the transesterification step proper, or it is necessary, on the other hand, to add a correspondingly larger quantity of the basic catalyst (cf. WO 92/00268).
This approach, however, leads to the consequence that on account of the required increased catalyst quantity additional costs are incurred and that hence rather substantial quantities of soaps are formed in the reaction mixture. As these compounds present surface-active properties as well they render the phase separation more difficult and must be separated, which gives again rise to further additional costs.
The present invention is therefore based on the problem of providing a method for the production of fatty acid esters of monovalent alkyl alcohols, which is as simple and economic as possible, which involves the lowest demands on the starting oils possible and which ensures, at the same time, high yields.
This problem is solved by a method for producing fatty acid esters of monovalent alkyl alcohols by base-catalyzed transesterification of triglycerides from natural or synthetic oils and/or fats containing free fatty acids and phosphatides as interfering accompanying substances, wherein initially the oils and/or fats are processed with an immiscible basic glycerin phase so as to neutralize the free fatty acids and cause them to pass over into the glycerin phase, and then upon separation of the glycerin phase by means of monovalent alcohols, the triglycerides are subjected to transesterification, using a base as catalyst, to form fatty acid esters, characterized in that after separation of the fatty acid esters, the basic glycerin phase produced during transesterification of the triglycerides is used for processing the oils and/or fats for removal of the free fatty acids, with the minimum quantity of catalyst used being calculated, relative to 1,000 g of the oil to be processed, as a function of the acid number of the oil and the mean molar mass of the oil, in correspondence with the equations (I) to (III):
with an acid number satisfying the inequation (I)
SZ less than (0.084 mol/1,000 g of oil)*M(KOH)*Yxe2x80x83xe2x80x83(I)
in correspondence with equation (II)
minimum quantity of cat/1,000 g of oil=0.088 mol/100 g oil*Yxe2x80x83xe2x80x83(II)
or else in correspondence with equation (III)
minimum quantity of cat/1,000 g oil=(SZ/M/KOH)*(0.088/0.084)xe2x80x83xe2x80x83(III)
with
Y=(880 g/mol)/(mean molar mass of the oil used) and
SZ=acid number of the oil used [(g KOH)/(1,000 g of oil)].